High stability pulse signal amplifier with inverse feedback



Dec. 8, 1964 M. D. DE MONG HIGH STABILITY PULSE SIGNAL AMPLIFIER WITH INVERSE FEEDBACK Filed March 22, 1957 TIAQ .0

riceIlDeMong INVENTOR.

A7 'J 'ORNEYS pulses.

United States Patent Office 3,160,820 Patented Dec. 8, ,1964

This invention relates to an improved pulse signal amplifier and especially to such an amplifier for use in the guidance control system of a guided missile.

In certain types of guided missiles guidance is controlled by means of a radiant-energy-responsive sighting unit carried on the missile itself. In one such type of guidance system the sighting unit is adapted, when properly aligned with 'the target, to generate two series of output pulses or signals, one series indicative of UP- DOWN alignment and the other of LEFT-RIGHT alignment of the sighting axis. Each series of pulses comprises pairs of alternate positive-going and negative-going pulses occurring at a relatively low repetition rate of about cycles per second. Absence of either pulse in a series is indicative of misalignment of the sighting axis in the corresponding plane and is utilized to initiate corrective movement of the sighting unit to realign the sighting axis with the target.

The strength of the individual pulses, even when the sighting axis is properly aligned, may vary over extremely wide limits depending upon such factors as the distance to, and the nature of the target. In order -to utilize these pulses for their desired purpose it is necessary that the weakest useable pulses be amplified up to a sufciently high level to insure proper operation of the correction apparatus, without overloading of either the amplifier or the correction apparatus on the strongest pulses, and while discriminating against unwanted signals such as stray electrical noise etc.

It is an object of this invention to provide such an amplifier, which not only attains these results most effectively but which uses relatively few components, and is extremely tolerant of normal variations in the characteristics of the individual components.

Further objects will become apparent from the following Specification and claims, especially when considered in the light of the accompanying drawings wherein:

FIG. l is a diagrammatic representation of the type of signals to be handled by my improved amplifier.

FIG. 2 is a block diagram of an amplifier embodying my invention.

FIG. 3 is a schematic diagram of the amplifier of FIG. 2.

Considering first FIG. l, a pulse series of the type which is representative of the signals to be handled by my improved pulse amplifier, is diagrammatically illustrated. As shown this pulse series normally consists (when the sighting axis is correctly oriented in that particular plane) of alternate positive-going and negativegoing pulses 10 and 11, each pair of pulses being repeated at one-tenth-second intervals.

It is desired to amplify these pulses to a quite considerable extent and to rectify or invert 'alternate pulses so that they are all of the same polarity. Where the amplifier is to be utilized in a missile guidance system of the type previously described it is important to sense the absence of either the positive-going or negative-going Analysis of the pulse series will show that it consists of a fundamental component of 10 cycles per second together with higher harmonics thereof, and'it i should benoted that the fundamental l0 cycle Vcomponent will be present even though one or the other of the positiveor negative-going series of pulses is absent.

Thus, it is desirable that the response of the associated amplifier be fairly highly attenuated at this basic 10 cycle frequency to avoid a false on course signal even though one type of pulses is missing. Moreover to avoid overshoot on rapidly rising, strong signals, as well as to differentiate against stray electrical noise it is desirable that the response curve of the amplifier drop ofi quite rapidly in the vicinity of cycles per second.

The amplifier described herebelow attains these results while still maintaining a high degree of stability in thev intermediate range of frequencies between these limits.

As shown in block-diagram form in FIG. 2 the signal amplifier comprises a three-stage inverse feedback ampliiier 20, the last stage of which provides balanced or push-pull output, a full-wave detector 21 responsive to the output of said feedback amplifier, a cathode follower 22 for applying the output of the detector to a variable threshold clipper or limiter 23 which in turn drives la low gain output stage 24.

The feedback amplifier 2t) comprises three triodes 2S, 26 and 27 connected in cascade and with a relatively high degree of overall inverse feedback. The cathodes 28 and 29 of triodes 25 and 26 are connected to ground, which constitutes the negative side of a suitable D.C. power supply (not shown) used with the amplifier, through a common cathode resistor 30, which is shown as the lower portion of a voltage divider across the power supply the upper portion being constituted by the relatively high resistance 31. The cathode 32 of triode 27 is connected to the cathodes 28 and 29 by a pair of individual resistors 33 and 34 the junction of which constitutes one output terminal of the feedback amplifier. Anode resistors 35, 36 and 37 are connected between the respective anodes of the triodes 25, 26 and 2'7 and suitable positive supply lines 38, 39, the other output from the feedback amplifier being taken from the anode of triode 2.7. Resistors 34 and 37 are of similar value, so that variations in anode current in tube 27 as a result of signals applied to the grid 41 thereof will produce substantially equal but opposite changes in the potential at the two output terminals of this stage whereby balanced, orpush-pull input signals may be applied to the following detector stage. In an actual amplifier built in accordance with my invention and which utilized the circuitry shown in FIG. 3, balance of these signals was improved by shunting resistor 37 with a relatively low value (.001 rnfd.) capacitor as clearly shown in the drawing. Such capacitor serves to balance the stray capacities effectively shunting resistor 34.

Input pulses applied to the kgrid 42 of triode 25 and amplified thereby arev coupled by the R-C network formed by coupling capacitor 43 and grid resistor 44 to the grid 45 of triode 26. Theresulting output pulses at the anode of triode 26 are similarly coupled by the R-C network formed by coupling capacitor 46 and grid resistor 47 to the grid 41 of phase splitter 27.

Multi-stage negative feedback amplifiers having appreciable overall gain have a tendency to become un-v attained with the use of a much smaller capacitor 48 of Y considerably lower voltage rating connected. directly be-4 tween the anodes of the first two stages 25 and 26. v

Stability in the low frequency region as well as attenuation of the unwanted ll0-cycle sine wave component ofthe input signals is attained byvproper proportioning- 3 of the coupling capacitors 43 and 46 relative -to one another. g As will be noted from FIG. 3 coupling networks formed by capacitors 43 and 46 and their associated resistors 44 and 47 effectively constitute high pass filters g which tend to attenuate the response of the ampliiierv to the lower frequencies. Moreover, by using a considerably larger value of capacitor at 46 than at 43 the tendency, which would otherwise obtain, for unwanted oscillation or appreciable resonance effects to occur at these low frequencies is effectively suppressed.

The two balanced outputs from the phase splitter 27 of the feedback amplifier are applied through coupling capacitors 49 and 50 to the full wave detector 21 shown as a duo-triode 51 having the grid and plate of each section tied together to serve as anodes 52 and 53, and with the cathodes 54 and 55 of both sections directly connected together and to a load resistor 54. As described in detail in my copending application Ser. No. 647,930, now Patent No. 2,912,584, entitled Full Wave Detector this detector is provided with a voltage dividing network 55 for applying a predetermined positive bias to each anode, so that current will be owing through one or both sections of the diode at all times, even in the absence of any input signal. As explained indetail in said copending application, this arrangementrenders the detector responsive to extremely weakinput signals. With the arrangement shownrboth the positive-and negative-going signal pulses applied to the input of feedback amplifier 20 will appear as amplified positive-going pulses at the cathode end of detector load resistor 54.`

These output pulses are applied directly -to the grid of cathode follower 22, the output of which is coupled through the R-C coupling network formed by capacitors 56 and 57 and the resistors 58 and 59 to the grid 60 of the variable threshold clipper 23.

Up to this point signals appearingat the input of the feedback amplifier and falling within the passband there- Y 23 also performs an additional function.

of will have all been amplified substantially the samek amount regardless of their original amplitude. Asprevviously indicated they may vary in amplitude over a very wide range, depending Yupon such factors as thel distanceto and the nature of the,V target being tracked. However it is the presence or absence of signal pulses, not their amplitude that determines'whether proper guidance is beingobtained and the guidance correcting system v is therefore arranged to respond to this condition, rather than to the amplitude.

The overall gain therefore need only be'suflicient vto I amplify the weakest signal pulses that it is desired to sary that the dynamic range of the output signals be con- Y siderably smaller than that of the'input signals, and to y obtain this result some'kind of automatic gain con-trol is cuits are notappropriate for this use in a guidance system due to theirl inherently slow response and recovery rates.

' pulse.

strongest signals where the bias is maximum only the extreme peaks will be permitted to pass. At the same time the maximum output signal will be limited by the relatively low anode voltage. Thus the dynamic range of amplitudeof the output'signals from limiter 23 will be much smaller than thatof the signals appearing at the input.

The network between cathode follower 22 and limiter Under centain conditions, for example where a relatively large target is being tracked at moderately close Vrange the input signal to the amplitier tends to become distortedA so that either the positive or negative pulse takes on a doublehumped characteristic. Since the signals will berelatively strong un'der these circumstances condenser 57 will become charged on the first humpr of such a distorted The time constant of condenser 57 and resistor 59 is sufciently long that the grid 60 will still be biased negatively at the arrival of the second hump and as a result the amplitude of this second hump at the output of the limiter will be considerably lower than and will tend to blend in to the lirst hump, with'the desirable result that but one effective output pulsel will occur even though the input pulse is thus ldistorted.

Output stage 24 is a relatively low gain stagewhose primary function is to provide a proper impedance match for the following control apparatusr (not shown) which will normally be connected to the amplilier output terminal V66 to which the output-of 'the signal amplifier is applied through coupling capacitor. 65.

' Operation of the invention isibelieved to have ,been made clear in conjunction with the above specific description of the invention. Pulse signals applied to the input 19 of the signal amplifier are first amplified in triodes' 25 and 26 and then passed through the phase splitter 27 to provide a pair of balanced output signals out of phasel with one another. These balanced signals arethen detected bythe full wave detector 21 wherein they are Vall converted to positive-goingpulses which "arey passed by way of the impedance matching cathode follower 22 to the variable gain clipping amplifier or limiter 23 which Vserves to amplify the weaker signals toa much greater extent than the stronger signals. From the output of the limiter the signals pass to the low gain output stage 24 and thence to the output terminal 66. Due to the relatively high Ydegree-of inverse feedback utilized, particularly in amplifier 20, the unit is tolerant of relatively wide-variations-in the characteristicsY of most ofthe components, including the vacuum tubes.

While obviously the exact values of the various cornponents will vary depending upon the desired frequency pass band, tubes used, etc., the values shown besidel each i componenten the schematic diagram, FIG. 3, have been required. The conventional automatic gain control cir- Y However'the variable threshold clipper or limiter ZSutifectively.

. lized in `this invention performs this function Vmost ef-H AsV shown in the schematic diagram this clipper-comprises atriode 61, with its anode voltage obtained from Yand scope ofthef'inventionas deined'by found most satisfactory'for the specific conditions chosen for purposes of illustration. In this specific' embodiment tube 25 is atype 5.719,y while'all other ,stages are constituted by one or both sections of type 6112 tubes.

' As Vwill be readilyV understood by thosefskilled in the claims.

I claim:

Y' 1. Afmulti-stage pulse amplifier comprising lirst,

Y anode resistors for connecting the anodeof each triode un-bypassed'cath'ode resistor 64 serves to normally bias Vthe stage for class A operation when the signalsy appear- 1 ing at the output of cathode follower 22 are kat a rela-V tively` low level.V However, stronger signalsnwill cause triode 6 1 to begin to Vdraw gridrcurrent through'resistors 58 and 59 'causing condenser 57 to acquire a negative Ycharge thereby increasing the biasvon the grid 61, This will clip Vthe lower portion @of ythe signal so that onjthe to a suitable sourcel of positive voltage, a common cathode resistor for `connecting-the cathode'sof said first and second'triodes to vthe lnegative side ofsaidrsource, means including a second,V cathode resistor Vconnecting network connecting.;V the anode of said-first triode to the grid-of said second triode, asecond Vresistance-capacity the o appendedV coupling network connecting the anode of said second stage to the grid of said third stage, means for applying input pulses t the grid of said rst triode, said second cathode resistor and the anode resistor of said third triode being of substantially similar resistance whereby each input pulse applied to said first triode will produce a pair of output pulses of opposite phase to one another across said last-mentioned resistors, a capacitor being connected directly between the anodes of said irst and second triodes whereby to attenuate the high frequency response of said ampliiier, and said coupling networks each having a relatively short time constant, but with the time constant of said second coupling network being considerable greater than that of said iirst coupling network.

2. A signal ampliiier for pairs of input pulses of opposite polarity and of relatively low repetition rate comprising a three-stage amplifier the last stage of which constitutes a phase-splitter, adapted to provide a pair of balanced output signals in phase opposition to one another in response to a pulse applied -to the input of said amplifier, said amplifier having relatively high overall inverse feedback and including first and second stages including triodes having anodes furnishing output terminals for said stages, a capacitor directly interconnecting the anodes of the first and second stages whereby to attenuate the high frequency output of said amplifier, a full-wave detector responsive to the balanced output signals from said phase splitter -to provide unidirectional pulses in response to said input pulses, and avariable threshold clipper amplifier including biassing means responsive to the amplitude of said unidirectional pulses applied thereto to vary the clipping level thereof in accordance with such amplitude whereby to eifectively compress the dynamic range of the resulting amplified pulses appearing at the output of said clipper amplifier.

References Cited in the ile of this patent UNITED STATES PATENTS 2,559,515 Pourciau July 3, 1951 

2. A SIGNAL AMPLIFIER FOR PAIRS OF INPUT PULSES OF OPPOSITE POLARITY AND OF RELATIVELY LOW REPETITION RATE COMPRISING A THREE-STAGE AMPLIFIER THE LAST STAGE OF WHICH CONSTITUTES A PHASE-SPLITTER, ADAPTED TO PROVIDE A PAIR OF BALANCED OUTPUT SIGNALS IN PHASE OPPOSITION TO ONE ANOTHER IN RESPONSE TO A PULSE APPLIED TO THE INPUT OF SAID AMPLIFIER, SAID AMPLIFIER HAVING RELATIVELY HIGH OVERALL INVERSE FEEDBACK AND INCLUDING FIRST AND SECOND STAGES INCLUDING TRIODES HAVING ANODES FURNISHING OUTPUT TERMINALS FOR SAID STAGES, A CAPACITOR DIRECTLY INTERCONNECTING THE ANODES OF THE FIRST AND SECOND STAGES WHEREBY TO ATTENATE THE HIGH FREQUENCY OUTPUT OF SAID AMPLFIER, A FULL-WAVE DETECTOR RESPONSIVE TO THE BALANCED OUTPUT SIGNALS FROM SAID PHASE SPLITTER TO PROVIDE UNDIRECTIONAL PULSES IN RESPONSE TO SAID INPUT PULSES, AND A VARIABLE THRESHOLD CLIPPER AMPLIFIER INCLUDING BIASSING MEANS RESPONSIVE TO THE AMPLITUDE OF SAID UNIDIRECTIONAL PULSES APPLIED THERETO TO VARY THE CLIPPING LEVEL THEREOF IN ACCORDANCE WITH SUCH AMPLITUDE WHEREBY TO EFFECTIVELY COMPRESS THE DYNAMIC RANGE OF THE RESULTING AMPLIFIED PULSES APPEARING AT THE OUTPUT OF SAID CLIPPER AMPLIFIER. 